Myeloperoxidase (MPO), a glycoprotein present in the azurophilic granules of human polymorphonuclear neutrophils (PMNs), is the most abundant protein in the mature cell, and is of critical importance both as a major bacteriocidal protein and in categorizing acute leukemias of the myeloid lineage. MPO activity is, therefore, an important histochemical marker in distinguishing myeloid from lymphoid leukemias. In addition, MPO deficiency is a common disorder whose pathogenesis and molecular basis are not known. MPO is synthesized in the promyelocytes, but it's subunit composition, genetic polymorphism, regulation of expression, post-translational processing and modification, and its amino acid sequence are not known. To date, no cDNA clones for MPO were available. We and others have found that human promyelocytes cell line HL-60 synthesizes MPO. We have constructed a cDNA library from HL-60 mRNA in the expression vector gt 11 and have screened this and a human bone marrow mononuclear cell library with MPO- specific antibodies. Several MPO cDNA clones were isolated and are being further characterized. cDNA clones for MPO will open the way for detailed molecular characterization of the structure and expression of this important protein. Full length MPO cDNA clones will be isolated. MPO mRNA will be characterized and the complete amino acid sequence of the protein will be determined from nucleotide sequencing of the MPO cDNA. The synthesis and accumulation of MPO mRNA during granulocyte differentiation will be investigated by RNA blotting of stage-specific PMN progenitors. The structure of the MPO gene will be studied in normal granulocytes and in primary acute leukemias by genomic DNA blot analysis and in situ hybridization.